Image Forming Apparatus

Abstract: An image forming apparatus includes a plurality of image carriers on each of which image is formed, a conveyor belt for carrying an image receiving medium, a driving roller on which the conveyor belt is mounted for driving the conveyor belt to convey the image receiving medium and a pressing roller for pressing the conveyor belt against the driving roller. The images formed on the image carriers are transferred seguentially to the image receiving medium conveyed by the conveyor belt.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to image forming apparatus which form images on an image receiving medium using a plurality of photosensitive drums such as a color copying machine, etc. 2. Description of the Related Art There is a color copying machine comprising four photosensitive drums arranged parallelly. On this type of copying machine, four photosensitive drums are arranged parallelly and toner images in different colors are formed on the respective photosensitive drums using yellow, magenta, cyanic and black toners. Each of these toner images is transferred and formed on a single sheet of paper. On the color copying machine using these four photosensitive drums, an image receiving medium placed on a conveyor belt are brought in contact with four photosensitive drums one by one and respective toner images are transferred from the drums onto the image receiving medium. Further, when forming an image other than color images, for instance, forming a black image only, no toner image is formed on three drums of yellow, magenta and cyanic drums and a black toner image is formed and transferred onto an image receiving medium. Thus, an image only in black is obtained. However, a conveyor belt is normally wound round driving rollers comprising rubber rollers and is moved by rotating the driving rollers. The largest reason for using rubber rollers is to prevent the conveyor belt from slipping against the driving rollers by making coefficient of statical friction of the rubber rollers with the conveyor belt large. Because, if the conveyor belt slips against the driving rollers, moving distances of copying papers being conveyed by the conveyor belt changes, causing a color shift on the image receiving medium in the conveying direction. That is, in order to prevent the conveyor belt from slipping against the driving rollers, it is desirable to use soft rubber rollers with hardness of rubber lowered. However, if a rubber roller is used, accuracy of the outer diameter of the driving roller drops and the more soft a rubber roller is, the more worse accuracy of the outer diameter of the driving roller will become. If accuracy of the outer diameter of the driving roller drops, the peripheral speed of the roller changes, making the conveying speed of the conveyor belt irregular and finally, a color shift is caused on copying papers in the conveying direction. When a conveyor belt is used for a long time, its surface becomes dirty as toners and paper powder of the image receiving medium attach thereon and therefore, the conveyor belt is cleaned with a belt cleaning device. However, this conveyor belt cleaning device cleans a belt by bringing a rubber blade in contact with the surface of the conveyor belt and a material having a high contact resistance against a rubber blade is used as for a conveyor belt. Therefore, when a conveyor belt is rubbed by a rubber blade of a belt cleaning device which is kept in contact with the conveyor belt, electric charge is left. Unless this residual electric charge is neutralized, residual potential of the conveyor belt becomes high and images are not satisfactorily transferred on the image receiving medium. Furthermore, such a problem is also caused that ozone is generated if a corona discharger is used to neutralize the residual electric charge. On this type of image forming apparatus, there was such a problem that the conveying speed of a conveyor belt becomes irregular as its peripheral speed changes if accuracy of the outer diameter of driving rollers drop and as a result, a color shift of images on an image receiving medium may be caused along the conveying direction of the image receiving medium. Further, as described above, the image receiving medium is conveyed toward four photosensitive drums by a conveyor belt. However, if the conveyor belt is moved while meandering unwillingly, the image receiving medium is also conveyed while meandering correspondingly and there was such a problem that the same images in different colors will be shifted as the images in different colors are transferred sequentially on the image receiving medium as a result of the meandering conveyance. In order to solve this problem, a regulation plate is provided at both ends of the rollers over which a conveyor belt is put as disclosed in the Japanese Utility Model Laid-open Publication (JITSU-KAI-HEI) 4-7543. The conveyor belt is moved while keeping its both ends in contact with these regulation plates to prevent the conveyor belt from meandering. In this construction, however, if a distance between two regulation plates provided at the rollers is not in accord with the width of a conveyor belt, a problem described below will be caused. That is, there will be such a problem that at a place where the distance between two control plates is wide, it is possible for the conveyor belt to meander and at a place where the distance between two control plates is narrow, the conveyor belt may possibly run over one of the regulation plates and as a result, a color shift will be caused on images on the image receiving medium along the direction perpendicular to the conveying direction of the image receiving medium. Further, on a conventional image forming apparatus, the rollers are rotated by transmitting the turning force of a motor to one of the rollers having parallel shafts over which a conveyor belt is put and a conveying force is provided by moving the conveyor belt in the rotating direction of the rollers. There was such a problem that if the moving speed of the conveyor belt becomes irregular, it is not possible to transfer images from four photosensitive drums at a prescribed position and as a result, a color shift is caused on images on the image receiving medium. In view of this problem, construction to use driving rollers directly as the rotary shaft of a motor without using driving transmission gears, etc. which may cause irregular moving speed of a conveyor belt. That is, a driving roller and a motor are in one united body. There are a belt cleaner, photosensitive drums, image transfer rollers, etc. arranged while kept in contact with this conveyor belt along its surface. These arrangements, however, will become loads when driving the conveyor belt. Further, when processing jammed image receiving medium, the conveyor belt is separated from the state in contact with the photosensitive drums and pulled out of the body of the apparatus. Because of this construction, in order to pull out the conveyor belt easily it is necessary to lower the belt to a location where the motor does not come in contact with the photosensitive drums. On the other hand, in order to drive a conveyor belt while overcoming loads, a motor needs a large torque. Generally, a motor in large size is used to improve its torque. However, because a roller and a motor for driving the conveyor belt are in one united body as described above, if a large motor is used, it becomes necessary to further lower the conveyor belt to prevent the photosensitive drums and the motor from contacting each other when processing jammed image receiving medium. Thus, there comes out a problem that the entire image forming apparatus will become large in size. SUMMARY OF THE INVENTION It is one of the objects of the present invention to provide an image forming apparatus which does not cause a color shift of images along the conveying direction of an image receiving medium. Another object of the present invention is to provide an image forming apparatus which does not become large in size even when a motor generating a large torque is used for driving rollers over which a conveyor belt is put. A further object of the present invention is to provide an image forming apparatus which does not cause a color shift of images along the direction perpendicular to the conveying direction of an image receiving medium. The objects are achieved by an image forming apparatus as claimed. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is an outline diagram of full color image forming apparatus according to the present invention applied; FIGURE 2 is a perspective view showing the first embodiment of the present invention less a part of the conveying means which is its essential part; FIGURE 3 is a graph showing a test result of difference in peripheral lengths and amount of skid movement of the conveyor belt; FIGURE 4 is a graph showing a test result of weighing and skid amount of the conveyor belt; FIGURE 5A through 5C are cross-sectional views showing the positional relation between the conveyor belt and the regulation belt; FIGURE 6 is a graph showing the state of skid movement of the conveyor belt when the construction of the first embodiment is not adopted; FIGURE 7 is a graph showing the state of skid movement of the conveyor belt when the construction of the first embodiment is adopted; FIGURE 8 is a perspective view showing the second embodiment of the present invention less a part of the conveying means which is its essential part; FIGURE 9 is a plan view of the second embodiment less a part of the conveying means; FIGURE 10 is a perspective view for explaining the skid movement of the conveyor belt in the second embodiment; FIGURE 11 is a front view for explaining the size and tapered state of a tapered roller used in the second embodiment; FIGURE 12 is a graph showing the state of skid movement of the conveyor belt when the construction of the second embodiment is not adopted; FIGURE 13 is a graph showing the state of skid movement of the conveyor belt when the construction of the second embodiment is adopted; FIGURE 14 is a perspective view showing the third embodiment less a part of the conveying means which is its essential part. FIGURE 15 is a plan view showing the third embodiment less a part of the conveying means; FIGURE 16 is a perspective view for explaining the skid movement of the conveyor belt in the third embodiment; FIGURE 17 is a graph showing the state of skid movement of the conveyor belt when the construction of the third embodiment is not adopted; FIGURE 18 is a graph showing the state of skid movement of the conveyor belt when the construction of the third embodiment is adopted; FIGURE 19 is a perspective view showing the fourth embodiment of the present invention less a part of the conveying means which is its essential part; FIGURES 20 through 30C are cross-sectional views showing the positional relation of the conveyor belt and the regulation plate; FIGURE 21 is a graph showing the state of skid movement of the conveyor belt when the construction of the fourth embodiment is not adopted; FIGURE 22 is a graph showing the state of skid movement of the conveyor belt when the construction of the fourth embodiment is adopted; FIGURE 23 is a perspective view showing the fifth embodiment of the present invention less a part of the conveying means which is its essential part; FIGURE 24 is a perspective view showing the sixth embodiment of the present invention less a part of the conveying means which is its essential part; FIGURE 25 is a perspective view showing the seventh embodiment of the present invention less a part of the conveying means which is its essential part; FIGURE 26 is a perspective view for explaining the skid movement of the conveyor belt in the seventh embodiment; FIGURE 27 is a graph showing the state of skid movement of the conveyor belt when the construction of the seventh embodiment is not adopted; FIGURE 28 is a graph showing the state of skid movement of the conveyor belt when the construction of the seventh embodiment is adopted; FIGURE 29 is a perspective view showing the eighth embodiment less a part of the conveying means which is its essential part; FIGURE 30 is a perspective view for explaining the skid movement of the conveyor belt in the eighth embodiment; FIGURE 31 is a graph showing the state of skid movement of the conveyor belt when the construction of the eighth embodiment is not adopted; and FIGURE 32 is a graph showing the state of skid movement of the conveyor belt when the construction of the eighth embodiment is adopted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the preferred embodiments of the present invention will be described in detail with reference to the drawings. First embodiment will be described with reference to FIGURE 1. FIGURE 1 shows the outline of the construction of a color copying machine as an image forming apparatus. In this color copying machine, four photosensitive drums 2Y, 2M, 2C and 2BK are arranged parallelly in this order as image carriers. Above these photosensitive drums, there are four image forming units 150Y, 150M, 150C and 150BK provided correspondingly for forming images on the respective photosensitive drums. Under these photosensitive drums there is a conveying means 200 provided for conveying an image receiving mediums 8, e.g. a sheet of paper, to the photosensitive drums 2Y, 2M, 2C and 2BK. Transfer rollers 5Y, 5M, 5C and 5BK are arranged corresponding to the photosensitive drums 2Y, 2M, 2C and 2BK as image transfer means for transferring toner images formed on the photosensitive drums onto image receiving medium 8 conveyed by the conveying means 200. Four sets of the image forming units 150Y, 150M, 150C and 150BK are composed of a recording unit comprising charging devices 3Y, 3M, 3C and 3BK, solid scanning heads 1Y, 1M, 1C and 1BK, developing devices 4Y, 4M, 4C and 4BK, cleaning devices 6Y, 6M, 6C and 6BK and discharging devices 7Y, 7M, 7C and 7BK respectively. Now, an yellow image forming unit 150Y will be described. The solid scanning head 1Y outputs exposure light to the photosensitive drum 2Y according to yellow image data being sent from a printing controller (not shown). The solid scanning head 1Y is in such a construction that it has very small light emitting sections arranged at equal spaces in the direction of the axis of rotation of the photosensitive drum 2Y, that is, on the line in the main scanning direction. Lighting of the individual light emitting sections on the line of the main scanning direction is controlled according to the on-off signals sent from a printing controller according to a pattern to be printed. A light image is exposed on the photosensitive drum 2Y corresponding to an original image from the light emitting sections on one for one basis. An LED head array of resolution 400 DPI was used for the solid scanning head 1Y. The charging device 3Y which charges the surface of the photosensitive drum 2Y, the developer device 4Y, the transfer device 5Y, the cleaning device 6Y and the discharging device 7Y are sequentially arranged around the photosensitive drum 2Y. The photosensitive drum 2Y is rotated and driven by a driving motor (not shown). The surface of the photosensitive drum 2Y is charged by the charging device 3Y which is composed of a conductive charging roller and provided in contact with the surface of the photosensitive drum 2Y. Further, the charging roller is rotating when kept in contact with the surface of the photosensitive drum 2Y. The surface of the photosensitive drum 2Y is formed by an organic photoconductor. Normally, this photoconductor has a high resistance but has a nature to change specific resistance of a lighted portion when light is applied. When light is applied to the charged surface of the photosensitive drum 2Y from the solid scanning head 1Y corresponding to a yellow print pattern, an electrostatic latent image of the yellow image pattern is formed on the surface of the photosensitive drum 2Y. The electrostatic latent image is a so-called negative latent image that is formed on the surface of the photosensitive drum 2Y through charging when specific resistance of the lighted surface of a photoconductor is dropped by the light applied from the solid scanning head 1Y to discharge electric charge on the surface of the photosensitive drum 2Y and on the other hand, electric charge of the portion to which no light was applied remains. Thus, the light from the solid scanning head 1Y forms an image at an exposing positional location on the charged photosensitive drum 2Y and the photosensitive drum 2Y with a latent image formed rotates to a developing position. Then, the latent image on the photosensitive drum 2Y is turned to a toner image as a visible image, by the developing device 4Y. The developing device 4Y contains a yellow toner that is containing a yellow dye and formed by resin. This yellow toner is friction charged when stirred in the developing device 4Y and has electric charge of the same polarity as that charged on the photosensitive drum 2Y. When the surface of the photosensitive drum 2Y passes through the developing device 4Y, the yellow toner is adhered electrostatically to the discharged latent image portion only and this latent image is developed by the yellow toner. The photosensitive drum 2Y with the yellow toner image formed on it is rotating continuously and the yellow toner image is transferred onto the image receiving medium 8 on the conveyor belt 12, that is timely fed by the transfer device 5Y which is in the transfer position. A paper supply means is composed of a pickup roller 9, a feed roller 10 and a register roller 11. The image receiving medium 8 taken out of a paper supply cassette 23 by the pickup roller 9 is conveyed to the register roller 11 by one sheet only by the feed roller 10. The register roller 11 feeds the image receiving medium 8 after properly correcting its position. The peripheral velocity of the register roller 11 and that of the conveyor belt 12 have been so set that they become equal to the peripheral velocity V0 of the photosensitive drum 2Y. The image receiving medium 8 is conveyed to the transfer position of the photosensitive drum 2Y together with the conveyor belt 12 at a predetermined velocity equal to that of the photosensitive drum 2Y while being partially kept by the resister roller 11. The yellow toner image on the photosensitive drum 2Y which is kept in contact with the image receiving medium 8 is removed from the photosensitive drum 2Y and transferred onto the image receiving medium 8 by the transfer device 5Y. As a result, the yellow toner image in a print pattern based on a yellow print signal is formed on the image receiving medium 8. The transfer device 5Y is composed of a semiconductive transfer roller. This transfer roller 5Y supplies an electric field having the polarity reverse to a potential of the yellow toner adhered statically to the photosensitive drum 2Y through the back side of the conveyor belt 12. This electric field acts on the yellow toner image on the photosensitive drum 2Y through the image receiving medium 8 and as a result, the yellow toner image is transferred onto the image receiving medium 8 from the photosensitive drum 2Y. The image receiving medium 8 with the yellow toner image thus transferred is conveyed sequentially to a magenta image forming unit 150M, a cyanic image forming unit 150C and further to a black image forming unit 150BK. Further, the magenta image forming unit 150M, the cyanic image forming unit 150C and the black image forming unit 150BK contain a magenta (M), cyanic (C) and black (BK) color developers, respectively, instead of a yellow (Y) developer contained in a developing device 4Y for the yellow image forming unit 150Y. As these image forming units are constructed from the same components and their operations are all the same, the explanations of these image forming units will be omitted to make the explanation simple. Now, the image receiving medium 8 with color images formed one over another while passing through the yellow, magenta, cyanic and black transfer positions is conveyed to a fixing device 13. The fixing device 13 is composed of a heat roller with a heater incorporated fixes the toner images in various colors on the image receiving medium 8 permanently by heating and fusing the color toners. The image receiving medium 8 with the fixed image is ejected on a receiving tray 15 by the exit roller 14. On the other hand, the photosensitive drums 2Y, 2C and 2BK in respective colors passed through the transfer positions are driven and cleaned by cleaning devices 6Y, 6M, 6C and 6BK to remove residual toners and paper powder on the drums. Further, the potentials on the surfaces of the photosensitive drums 2Y, 2M, 2C and 2BK are regulated to a certain level. Then, a series of image forming processes from the charging devices 3Y, 3M, 3C and 3BK will begin. After conveying the image receiving medium 8 to the fixing device 13, the conveyor belt 12 is cleaned by a cleaning device 22 to remove residual toners and paper powder adhered to the surface of the belt and conveys next image receiving medium 8 when required. Further, in the case of a unicolor print, the image forming by an image forming unit in a desired unicolor is carried out. At this time, other image forming units in colors other than the selected color do not perform their operations. Next, a conveying means 200 6 in the first embodiment will be described with reference to FIGURES 2 to 6. FIGURE 2 shows the outline of the construction of a conveying means 200 6 . Reference number 12 6 shows a conveyor belt, 16 6 shows a driving roller, 17 6 shows a driven roller, 46 6 shows a regulation belt, 18 6 A and 18 6 B show a first compression spring and a second compression spring to give a tensile force to the conveyor belt 12 6 , and 21 6 shows a driven roller bearing. The regulation belt 46 6 is mounted or formed along inner side at one end of the conveyor belt 12 6 . The endless type conveyor belt 12 6 is driven by the driving roller 16 6 and the driven roller 17 6 . The driven roller 17 6 gives a tensile force to the conveyor belt 12 6 when its bearing 21 6 is pressed by the first and the second compression springs 18 6 A and 18 6 B. When a cause for generating a skid of the conveyor belt 12 6 was investigated to reveal that it was largely affected by a difference in pressures generated by the first and the second compression springs 18 6 A and 18 6 B. The results of this test are shown in FIGURES 3 and 4. FIGURE 3 shows the test result of amounts of skid per one turn of an endless type conveyor belt which was prepared by cutting a belt into several pieces in trapezoidal shape intentionally giving different peripheral lengths and connecting their ends to an endless conveyor belt. The axis of abscissa shows differences in peripheral lengths at the ends of a belt and the axis of ordinate shows amount of skid per one turn of the belt. In this test, for the purpose of making clear an effect of only peripheral length of the belt, a precisely prepared weight is used for giving a tensile force to the belt. Further, the shorter peripheral length side was made as the plus side of skid direction of the belt. As a result, it is seen that the more larger a difference in peripheral lengths becomes, the more larger the skid becomes. Furthermore, it is also seen that the skid progresses at the shorter peripheral length side of the belt. On the other hand, shown in FIGURE 4 is amount of skid per one turn of the belt measured by changing a difference in loads applied at both sides, and a difference in spring loads generating a tensile forces is shown. The axis of abscissa shows differences in spring loads generating tensile force and the axis of ordinate shows amount of skid per one turn of the belt on the axis of ordinates. The graph in FIGURE 4 shows "Difference in Spring Loads Generating Tensile Force". In this test, for the purpose of conducting the test by making load difference clear, a precisely prepared weight was used. Further, for the purpose of investigating an effect of load difference only, a belt manufactured precisely in micron unit on an experimental bases was used. Further, the side of the belt having a larger tensile force generating spring load applied was made as the plus side of skid direction of the belt. As a result, it is seen that the more larger a load difference becomes, the more larger the degree of skid becomes correspondingly. Further, it is also seen that the skid of the belt progresses at the side with a larger belt tensile force generating spring load. Now, these two test results can be summarized as follows: (1) The skid of the belt progresses at the short peripheral length side. (2) The skid of the belt progresses at the large load side. On the other hand, it is impossible to make the peripheral lengths of the conveyor belts 12 6 completely equal on all actual apparatus. Further, it is also impossible to completely eliminate fluctuations of the first and the second compression springs 18 6 A and 18 6 B. It was decided to control the direction of skid of the conveyor belt 12 6 based on the above results in this embodiment. That is, as illustrated in FIGURE 2, the endless type conveyor belt 12 6 put over the driving roller 16 6 and the driven roller 17 6 is made in the construction having a difference in its peripheral lengths at both sides of L1>L2 when the peripheral lengths at both sides are L1 and L2. As a means for giving a tensile force to the conveyor belt 12 6 , a tensioning mechanism 210 6 is composed of a first and a second compression springs 18 6 A and 18 6 B which are a first and a second tensioning members. That is, the first compression spring 18 6 A having a strong pressure P1 is arranges at the shorter peripheral length L2 side of the conveyor belt 12 6 and the second compression spring 18 6 B having a weak pressure P2 (P1>P2) is arranges at the longer peripheral length L1 side. As a result of this construction, the conveyor belt 12 6 skids always to the first compression spring 18 6 A side having a strong pressure P1 at the shorter peripheral length L2 side. On the other hand, an skid preventive guide 47 6 is provided along the peripheral edge of the conveyor belt 12 6 with the second compression spring 186B having a weak pressure P2 arranged at the longer peripheral length L1 side. And, by bringing this regulation belt 46 6 in contact with the end of the driven roller 17 6 (or the driving roller 16 6 ), the skid of the conveyor belt 12 6 is prevented. The construction of this regulation belt 46 6 is as shown in FIGURES 5A to 5C. That is, this regulation belt 46 6 is in the thick belt shape and provided along the back side of the peripheral edge of the conveyor belt 12 6 with the second compression spring 18 6 B arranged. As the conveyor belt 12 6 always skids to the first compression spring 18 6 A side having the strong pressure P1 at the shorter peripheral length L2 side, after a time "t" passed shown in FIGURE 5B from the initial state shown in FIGURE 5A, the regulation belt 46 6 runs against the end of the driven roller 17 6 to prevent the further movement of the conveyor belt, which is then brought in the balanced state. FIGURE 6 shows the result of the skid of the conveyor belt when the measures described above were not taken and FIGURE 7 shows the result of the skid of the conveyor belt when the measures described above were taken. As the results of this test, running times of the belt shown in "Test Time (Second)" are plotted on the axis of abscissas and "Running Position ( µ m)" showing amounts of the skids of the belt are plotted on the axis of ordinates. As clear from this test results, amount of the skid of the belt which was traveled without setting its mounting and pressure was large, the color shift of images on the image receiving medium 8 tends to occur in the direction perpendicular to the moving direction of the conveyor belt 12 6 . However, the skid of the conveyor belt is very small when the belt was traveled with its mounting and pressure set, and it can be seen that the conveyor belt 12 6 was in the stable running state scarcely causing the color shift of images on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt 12 6 . The test results shown in FIGURES 6 and 7 are one example. The further statistic test revealed that the same effect is obtained up to a difference in peripheral lengths 2 mm of both sides of a belt if a difference in pressures applied is suppressed to accuracy of 1 kg according to the construction in the first embodiment. Accuracy of length ±0.01 mm and pressure ±50 g was demanded for conventional belt and therefore, when a belt in this construction is used, it is possible to effectively control and restrain the skid direction without demanding high accuracy. As described above, the conveying means in the first embodiment is capable of controlling the skid of the conveyor belt 126 in a very simple construction. Next, a conveying means 200 7 in the second embodiment will be described with reference to FIGURES 8 to 13. As illustrated in FIGURES 8 and 9, a tapered roller 17 7 is used as a driven roller. This roller is tapered so that its diameter is increased gradually to a large diameter from one end to another end. The regulation belt 46 7 is positioned at the small diameter side of the tapered roller 17 7 and mounted along the back side of the peripheral edge of a conveyor belt 12 7 in the same manner as in FIGURES 5A to 5C. When the conveyor belt 12 7 is put over driving rollers 16 7 and the tapered rollers 17 7 which are driven rollers, the conveyor belt 12 7 skids toward the large diameter of the tapered roller 17 7 . In this case, on the conveyor belt 12 7 being pulled along the tapered roller 17 7 , a tensile force F acting in the vertical direction is first generated on its inclined portion, which is above the inclined portion of the tapered roller 17 7 as illustrated in FIGURE 10. When the conveyor belt 12 7 is moving, the tensile force F is divided into FH in the belt conveying direction and F V in the vertical direction and these divided forces act on the conveyor belt. The direction F V vertical to the conveying direction of the belt is the direction toward the large diameter of the tapered roller 17 7 and the conveyor belt 12 7 is moved one-sidedly toward the direction of the large diameter of the tapered roller 17 7 by this force F V . That is, the direction of the skid of the conveyor belt 12 7 can be controlled using the tapered roller 17 7 as a driven roller. If the direction of the skid can be controlled, a single piece of the guide 46 7 is sufficient to restrain progress of the skid. That is, it can be achieved by providing the regulation belt 46 7 only at the inside of the conveyor belt 12 7 at its small diameter side. That is, the conveyor belt 12 7 skids toward the large diameter side but when the conveyor belt 12 7 moves one-sidedly for a certain amount, the skid preventive guide 46 7 is slided to the roller end surface of the small diameter side of the tapered roller 17 7 , stopping the further skid at a position where the skid force of the conveyor belt 12 7 is balanced with the rubber repulsive force of the guide 46 7 . Once these forces are balanced each other, the conveyor belt 12 7 is moved continuously in this balanced stated. FIGURE 11 shows a definite dimensional relation of the shape of the tapered roller 17 7 and the conveyor belt 12 7 which were used in the second embodiment. That is, the tapered roller 17 7 is 260 mm long and the conveyor belt 12 7 put on this tapered roller 17 7 is 258 mm wide. The diameter of the large diameter portion of this tapered roller 17 7 is 22.3 mm and that of the small diameter portion is 21.9 mm. Therefore, as shown by the following expression, this tapered roller 17 7 has a taper of 0.001538. 22.3-21.9/260 = 0.001538 FIGURE 12 shows the test result of skid of the conveyor belt when no measures described above were taken and FIGURE 13 shows the test result of skid of the conveyor belt when the measures described above were taken. As the result of this test, "Test Times (Sec.)" showing the running times of the conveyor belt were plotted on the axis of abscissas and "Running Positions ( µ m)" showing amount of skid of the conveyor belt were plotted on the axis of ordinates. Therefore, the skid of the conveyor belt when it was moved without taking any measure is large while the color shift of images on the image receiving medium 8 tends to occur in the direction perpendicular to the moving direction of the conveyor belt 12 7 . However, it is seen that the skid of the conveyor belt when it was moved with the tapered roller 17 7 and the regulation belt 46 7 provided is very small and the belt ran in the stable state scarcely causing the color shift of images on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt 12 7 . The tapered roller 17 7 shown in this second embodiment is not needed to be applied as a driven roller, and when used as a third roller other than the driving roller 16 7 and the driven roller 17 7 , its effect will not be changed. Further, it is also not required to have the tapered roller 17 7 act from the inside of the conveyor belt 12 7 and its effect is not changed even when it was acted on the surface of the conveyor belt 12 7 . Further, in this second embodiment the tapered roller 17 7 was described as a driven roller and its small diameter side end surface was explained as the surface contacting the regulation belt 46 7 . However, not limited to these usages, the end surface of the driving roller 16 7 may be used as the skid prevention surface and even when a roller having an original skid prevention surface is provided, its effect will not be changed. As described above, the skid of the conveyor belt 12 7 can be controlled by a mechanism in very simple construction. Next, a conveying means 200 8 in the third embodiment will be described with reference to FIGURES 14 to 18. As illustrated in FIGURES 14 and 15, between the driving roller 16 8 and the driven roller 17 8 arranged parallel to each other, there is a diagonal roller 50 8 arrange diagonally to these rollers 16 8 and 17 8 . That is, it is arranged so that its one end 50 8 A is close to the driven roller 17 8 and another end 50 8 B is close to the driving roller 16 8 . Further, this diagonal roller 50 8 is arranged slightly below the plane surface connecting a driving roller 16 8 and a driven roller 17 8 and functions as a skid moving direction control roller. A conveyor belt 12 8 is put over these driving roller 16 8 , the diagonal roller 50 8 and the driven roller 17 8 . On the other hand, a regulation belt 46 8 is provided along the side edge of the conveyor belt 12 8 having a longer distance between the driving roller 16 8 and the diagonal roller 50 8 . The regulation belt 46 8 is in the construction as illustrated in FIGURES 5A to 5C. On the conveying means 200 8 in this construction, when moved, the conveyor belt 12 8 progressively skids toward the end having a shorter distance between the diagonal roller 50 8 and the driving roller 16 8 , that is, the conveyor belt 12 8 skids to the end 50 8 B of the diagonal roller 50 8 . As illustrated in FIGURE 16, the conveyor belt 12 8 is first twisted by the diagonal roller 50 8 and a tensile force F is generated in the direction vertical to the central axis of rotation of the diagonal roller 50 8 . In actual operation, this force F is divided into two forces which act in the belt conveying direction F H and in the direction F V vertical to the belt conveying direction. The direction F V of the divided force is the direction for the shorter distance between the diagonal roller 50 8 and the driving roller 16 8 and by this force, the conveyor belt 12 8 is given a force to move skiddingly in the direction of a shorter distance between the diagonal roller 50 8 and the driving roller 16 8 . That is, the conveyor belt 12 8 skids to the end 50 8 B side of the diagonal roller 50 8 . That is, it is possible to control the direction of skid of the conveyor belt 12 8 by providing the diagonal roller 50 8 which is not parallel to the driving roller 16 8 . If the direction of skid of the conveyor belt can be controlled, a single piece of the regulation belt 46 8 which controls progress of the skid is able to create its effect. That is, this is achieved when the guide 46 8 is provided only at the inside of the conveyor belt edge which has a long distance between the diagonal roller 50 8 and the driving roller 16 8 . That is, the conveyor belt 12 8 skids to the side with a shorter distance between the diagonal roller 50 8 and the driving roller 16 8 according to the diagonal roller 50 8 . However, if the conveyor belt 12 8 moved skiddingly by a certain amount, the regulation belt 46 8 slides to the end surface of the driven roller 17 8 and the skid of the conveyor belt is stopped at a position where the skid moving force of the conveyor belt 12 8 is balanced with the rubber repulsive force of the regulation belt 46 8 . Once both forces are balanced each other, the conveyor belt 12 8 continuously moves in this balanced state. FIGURE 17 shows the test result of the skid of the conveyor belt when no measures described above was taken and FIGURE 18 shows the test result when the measures described above were taken. As the result of this test, "Test Times (Sec.)" showing the running times of the conveyor belt were plotted on the axis of abscissas and "Running Positions ( µ m)" showing the amounts of the skids of the conveyor belt were plotted on the axis of ordinates. Therefore, the skid of the conveyor belt without taking no measure is large and the color shift of the images on the image receiving medium 8 tends to occur in the direction perpendicular to the moving direction of the conveyor belt 12 8 . However, the skid of the conveyor belt is very small when it was moved with the diagonal roller 50 8 and the regulation belt 46 8 provided and it can be seen that the conveyor belt 12 8 was running in the stable state scarcely causing the color shift on the images on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt 8. In this third embodiment, the diagonal roller 50 8 was arranged at the loose side of the conveyor belt 12 8 . However, the effect of the diagonal roller 50 8 does not change even when the diagonal roller 50 8 is arranged at the tension side of the conveyor belt if a space is available. Further, it is not necessary to have the diagonal roller 50 8 act from the inside of the conveyor belt 12 8 and its effect does not change even when the diagonal roller 50 8 is forced to act on the surface of the conveyor belt 12 8 . Further, the end surface of the driven roller 17 8 has been explained to be the surface contacting the regulation belt 46 8 in this third embodiment. However, the end surface of the driving roller 16 8 may be used as the skid control surface or when a roller having an original skid control surface is provided separately, its effect does not change at all. As described above, the skid of the conveyor belt 12 8 can be controlled by a system in very simple construction. Next, a conveying means 200 9 in the fourth embodiment will be described with reference to FIGURES 19 to 22. As illustrated in FIGURE 19, the conveying means 200 9 is in the construction of L1>L2 when the peripheral lengths of both edges of an endless conveyor belt 12 9 put over the driving roller 16 9 and the driven roller 17 9 are L1 and L2. As a means to give a tension to the conveyor belt 12 9 , a tensioning mechanism 210 9 is provided, which is composed of a first and a second compression springs 18 9 A and 18 9 B as a first and a second tensioning members, respectively. That is, the first compression spring 18 9 A having a strong pressure P1 is arranged at the L2 side of a short peripheral length of the conveyor belt 12 9 and the second compression spring 18 9 B having a weak pressure P2 (P1>P2) is arranged at the L1 side of the long peripheral length. As described in the first embodiment, as a result of this construction, the conveyor belt 12 9 always skids toward the length L2 side where the compression spring 18 9 A side having a strong pressure P1 is arranged. On the other hand, a regulation plate 41 9 is provided along the edge of the conveyor belt 12 9 with the compression spring 18 9 A having a strong pressure P1 at the L2 side of a short peripheral length. The regulation plate 41 9 kept in contact with the edge of the conveyor belt 12 9 prevents the skid of the conveyor belt 12 9 . That is, as illustrated in FIGURES 20A to 20C, the regulation plate 41 9 is arranged to penetrate the rotary shaft of the driving roller 16 9 . As the conveyor belt 12 9 always skids toward the first compression spring 18 9 A having a strong pressure P1 at the L2 side of a short peripheral length, after elapsing "t" time shown in FIGURE 20B, the edge of the conveyor belt 12 9 runs against the surface of the regulation plate 41 9 , preventing the further movement of the conveyor belt 12 9 and the conveyor belt 12 9 is kept in the balanced state. FIGURE 21 shows the state of skid of the conveyor belt when it was run without the belt mounting and pressure setting made as described above and FIGURE 22 shows the same when the conveyor belt was run with the belt mounted and pressure setting made as described above. As the results of this test, "Test Times (Sec.)" showing the running time of the conveyor belt is plotted on the axis of abscissas and "Running Positions ( µ m)" showing amount of skid of the belt is plotted on the axis of ordinates. As clear from this test results, amount of the skid of the conveyor belt is large when it was run without belt mounting and pressure setting made as described above and the color shift of the images on the image receiving medium 8 tends to occur in the direction perpendicular to the moving direction of the conveyor belt 12 9 . However, it can be seen that it is very small when the belt was run with the belt mounting and pressure setting made as described and the conveyor belt was in the stable running state with scarcely causing the color shift of the image on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt 12 9 . The test results shown in FIGURE 21 and 22 are only one example. Further statistical tests conducted revealed that the same results are obtainable according to the construction of the conveying means in this fourth embodiment if a difference in peripheral lengths of both side edges of the belts is suppressed to 1.5 mm and a difference of pressures applied is suppressed to 0.8 kg. As for accuracy of the conveyor belt, ±0.01 mm for length and ±50 g were so far demanded and therefore, when this construction is used, it is possible to effectively control and restrain the direction of skid without demanding high accuracy for the conveyor belt. FIGURE 23 shows a conveying means 200 10 in the fifth embodiment. In order to make the edges of a conveyor belt 12 10 and a regulation plate 41 10 easy to slide, a surface 43 10 teated with a low frictional resistance is provided in their contacting area. A test result of frictional resistance of an unprocessed stainless steel plate with a PET film was 0.665. On the other hand, coefficient of friction of an ordinary iron plate with fluorine coated is 0.657 and therefore, it is possible to obtain an equivalent coefficient of friction from a fluorine coated iron plate even when an expensive stainless steel having a low frictional surface resistance is not used. Further, needless to say, a lower coefficient of frictional resistance can be obtained if a stainless steel is coated with fluorine. FIGURE 24 shows a conveying means 200 11 in the sixth embodiment and a sheet 44 11 having a low coefficient of friction is inserted between a skid control plate 41 11 and the edge of a conveyor belt 12 11 . The sheet 44 11 of a low coefficient of friction is in somewhat large size and fixed to the skid control plate 41 11 by fixing adhesive tape 45 11 . Further, the method for fixing the sheet 44 11 is not restricted and any other method can be used. In the embodiments four to sixth regulation plates 41 9 to 41 11 are provided to the driving rollers 16 9 to 16 11 but they may be provided to the driven rollers 17 9 to 17 11 or along the entire edge of the conveyor belts 12 9 , to 12 11 . As described above, in the fourth to the sixth embodiments, an effective control of skid of the conveyor belt can be achieved when the conveyor belt 12 9 to 12 11 is so arranged that the conveyor belt is running while at least a part of it is kept in contact with the regulation plate 41 9 to 41 11 . Next, a conveying means 200 12 in the seventh embodiment will be described with reference to FIGURES 25 to 28. As illustrated in FIGURES 25 and 28, a tapered roller 17 12 of which diameter becomes larger gradually from one end to another end is used as a driven roller. A regulation plate 41 12 is provided along one edge of a driving roller 16 12 at the same side as the large diameter side of the tapered roller 17 12 . When the conveyor belt 12 12 is put over the driving roller 16 12 and the tapered roller 17 12 , which is a driven roller, the skid will progress toward the larger diameter of the tapered roller 17 12 when the conveyor belt is moved as described in the second embodiment. That is, as illustrated in FIGURE 26, a tensile force F vertical to the inclined portion that is the tapered portion of the tapered roller 17 12 is first generated on the conveyor belt 12 12 being pulled along the tapered roller 17 12 . When the conveyor belt 12 12 is moving, this tensile force F is split into two: F H acting in the belt conveying direction and F V acting in the direction vertical to the belt conveying direction. The direction F V of the split force vertical to the belt conveying direction is the direction toward the larger diameter of the tapered roller 17 12 and by this force F V , the conveyor belt 12 12 is moved one-sidedly in the direction of the larger diameter of the tapered roller 17 12 . That is, the direction of skid of the conveyor belt 12 12 is controlled using the tapered roller 17 12 as a driven roller and the movement is regulated by the regulation plate 41 12 provided at the larger diameter side of the tapered roller 17 12 . When the skid of the conveyor belt 12 12 progressed to a certain amount, the regulation plate 41 12 and the outer edge of the conveyor belt slide and the skid is stopped at a position where the skid moving force of the conveyor belt 12 12 is balanced with a reactive force of the regulation plate 41 12 . Once both forces are balanced, the conveyor belt 12 12 is continue moved in this balanced state. FIGURE 27 shows the test result of the skid moving state when the conveyor belt was run with no measure taken and FIGURE 28 shows the test result of the skid moving state when the conveyor belt was run with the tapered roller 17 12 and the regulation plate 41 12 provided. As the results of this test, "Test Times (Sec.)" showing running times of the conveyor belt is plotted on the axis of abscissas and "Running Position ( µ m)" showing the amount of skid of the belt is plotted on the axis of ordinates. As can be seen from these test results, the amount of skid of the conveyor belt is large and the color shift of the images on the image receiving medium 8 tends to occur in the direction perpendicular to the moving direction of the conveyor belt when no measure was taken. But, the amount of skid is very small when the conveyor belt 12 12 was run with the tapered roller 17 12 and the regulation plate 41 12 provided and the conveyor belt is in the stable running state without scarcely causing the color shift of the images on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt. The tapered roller 17 12 shown in the seventh embodiment is not necessarily to be used as a driver but can be used as a third roller other than the driving roller 16 12 and the driven roller as its effect will not be changed. Further, it is also not necessary to have the tapered roller 17 12 act from the inside of the conveyor belt and its effect will not be changed even when it is acted on the surface side of the conveyor belt 12 12 . As described above, it is possible to efficiently suppress the skid of the conveyor belt by a system in very simple construction. Next, a conveying means 200 13 in the eighth embodiment with reference to FIGURES 29 to 32. As illustrated in FIGURES 29 and 30, there is a diagonal roller 50 13 provided between a parallelly arranged driving roller 16 13 and a driven roller 17 13 not parallelly but diagonally to these rollers 16 13 and 17 13 . That is, the diagonal roller is so arranged that one end 50 13 A of the diagonal roller 50 13 A is close to the driven roller 17 13 side and another end 50 13 B is close to the driving roller 16 13 . Furthermore, this diagonal roller 50 13 is arranged at a position somewhat below the plane surface connecting the driving roller 16 13 and the driven roller 17 13 and functions as a skid control roller. The conveyor belt 12 13 is put over the driving roller 16 13 , the diagonal roller 50 13 and the driven roller 17 13 . On the other hand, a regulation plate 41 13 is provided along one side edge of the conveyor belt where a distance between the diagonal roller 50 13 and the driving roller 16 13 is short. The regulation plate 41 13 is in the construction as illustrated in FIGURES 20A to 20C. In the construction described above, the conveyor belt 12 13 moves one-sidedly toward the end of the diagonal roller 50 13 of which distance to the driving roller 16 13 is short. That is, the conveyor belt 12 13 moves one-sidedly toward the end 50 13 B of the diagonal roller 50 13 . In this case, as illustrated in FIGURE 30, the conveyor belt 12 13 is first twisted by the diagonal roller 50 13 and a tensile force F is generated in the direction perpendicular to the central axis of rotation of the diagonal roller 50 13 . In actual operation, this force F is split and acts in the belt conveying direction FH and the direction F V vertical to the belt conveying direction. The direction F V of a force split in the direction vertical to the belt conveying direction is a direction of a short distance of the diagonal roller 50 13 to the driving roller 16 13 and by this force the conveyor belt 12 13 is given a force to move one-sidedly in the direction of a short distance of the diagonal roller 50 13 to the driving roller 16 13 . That is, the conveyor belt 12 13 moves skiddingly to the end 50 13 B side of the diagonal roller 50 13 . That is, it is possible to control the skid direction of the conveyor belt 12 13 by providing the diagonal roller 50 13 which is not parallel to the driving roller 16 13 and to control the further skid by the regulation plate 41 13 . In other words, the conveyor belt 12 13 moves skiddingly to the short distance side between the diagonal roller 50 13 and the driving roller 16 13 following the diagonal roller 50 13 but when the conveyor belt 12 13 moves skiddingly to a certain distance, the outer peripheral edge of the conveyor belt slides on the regulation plate 41 13 and the skid of the belt is stopped at a position where the skidding force of the conveyor belt 12 13 is balanced with the reaction of the regulation plate 41 13 . Once both forces are balanced, the conveyor belt 12 13 moves continuously while kept in this balanced state. FIGURE 31 shows the test result of the skid of the conveyor belt when the measures described above were not taken and FIGURE 32 shows the same with the measures described above taken. As the results of this test, "Test Time (Sec.)" showing the belt running times is plotted on the axis of abscissas and "Running Positions ( µ m)" showing amount of skid of the belt is plotted on the axis of ordinates. Therefore, skid of the conveyor belt arranged without taking any measure is large and the color shift of the images tends to occur on the images on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt 12 13 . However, the skid of the conveyor belt 12 13 is very small when the diagonal roller 50 13 and the regulation plate 41 13 are arranged and it is seen that the conveyor belt 12 13 is in the stable running state scarcely causing the color shift of the image on the image receiving medium 8 in the direction perpendicular to the moving direction of the conveyor belt. In the eighth embodiment, the diagonal roller 50 13 was arranged at the loose side of the conveyor belt 12 13 . However, the effect of the diagonal roller 50 13 will not be changed even when it is arranged at the stretched side of the conveyor belt 12 13 if a space is available. Further, it is not necessary to have the diagonal roller 50 13 act form the inside of the conveyor belt 12 13 and the effect of the diagonal roller 50 13 does not change when the diagonal roller 50 13 is forced to act on the surface side of the conveyor belt 12 13 . As described above, it is possible to suppress the skid of the conveyor belt 12 13 by a system in very simple construction.

Claims: An image forming apparatus comprising: image carriers (2) on which different images are formed; a conveyor belt (12) for transporting an image receiving medium (8), means for transferring the images from the image carriers (2) to the image receiving medium (8) conveyed by the conveyor belt, the conveyor belt having a first length L1 measured along one peripheral edge and a second, shorter length, L2 measured along the other peripheral edge; a plurality of rollers (16, 17) on which the conveyor belt is mounted for moving the conveyor belt to convey the image receiving medium sequentially to a plurality of image carriers on which images are formed; tensioning means (18a, 18b) for supplying a predetermined tension to the conveyor belt; and a regulation member (46) for limiting a skid of the conveyor belt, characterised in that the rollers comprise: a driving roller (16), which has equal diameters at both ends, arranged perpendicular to the moving direction of the conveyor belt, and a following roller (17) opposed to the driving roller, and also characterised in that'the tensioning means includes a first spring member (18a) and a second spring member (18b) contacted with the following roller (17) respectively, the following roller (17) having a first portion supporting the longer edge of the conveyor belt and a second portion supporting the shorter edge, the first spring member (18a) applying a first pressure P2 to the first portion of the following roller (17) and the second spring member (18b), applying a second pressure P1 which is greater than the first pressure P2 to the second portion of the following roller (17).

Inventors: TODOME TUYOSHI, TODOME, TUYOSHI, Todome, Tuyoshi, Toshiba KK,Intell.Prop.Div.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1994
Main classification: G03G15
IPC codes: G03G15